Phase equilibria and thermodynamic evaluation approximating short-range ordering energy in the Fe-Rh binary system

Abstract

Phase equilibria between the a (A2), a' (B2) and y (A1 ) phases in the Fe-Rh binary system were investigated using electron probe micro-analysis (EPMA) and differential scanning calorimetry (DSC) techniques. The A2/B2 order-disorder transformation temperature was also examined by a transmission electron microscope (TEM) observations and high-temperature X-ray diffraction (HTXRD). It was confirmed that while the transition temperature from the a (A2) phase to the y (A1) phase decreases with increasing Rh content up to about 20at% Rh, the b.c.c. region is stabilized by further Rh addition in the composition range between about 20 and 50at% Rh, and then again the stability of b.c.c. phase decreases in the composition region over 50at% Rh. It was also shown by TEM observation and HTXRD examination that the compositions of the phase boundary of the A2/B2 ordering at 500°C and 600°C were determined to be 16.5 and 19.1 at% Rh, respectively. On the basis of those experimental results, a thermodynamic analysis was carried out. The results of the thermodynamic calculation suggest that the anomalous behavior of the stability of the b.c.c. phase is caused by the A2/B2 ordering. © 2009 ISIJ.